Project V (Mucus Dehydration and Evolution of COPD Lung Disease;R. Boucher, P.I.) will test the hypothesis that a significant component of the chronic bronchitic phenotype of COPD reflects the relative dehydration of airway mucus (secretions), which produces mucus adhesion to airway surfaces, infection of mucus with bacterial communities, inflammation, and airflow obstruction. Sp. Aim 1 will test this hypothesis directly in cross-sectional cohorts of Gold 0/1,2, and 3 subjects, measuring: 1) COPD mucociliary (MCC) and cough clearance (CC);2) mucus hydration (water content/activity);3) the concentrations of the dominant regulators of airway hydration, i.e., purine nucleotides (ATP) and nucleosides (ADO);and 4) the consequences of mucus dehydration on mucus biophysical properties and bacterial infection. Sp. Aim 2 tests the hypotheses that: 1) COPD acute exacerbations (AEs) reflect a transient worsening/failure of the mucus clearance (ciliary/cough-dependent) mechanism;2) the COPD patient is vulnerable to triggers of AEs because mucus clearance is chronically compromised by cigarette smoke-induced mucus dehydration;and 3) respiratory viruses trigger many COPD AEs via direct infection of the lower airway epithelium and derangement of the extracellular ATP/adenosine and cytokine pathways that regulate salt/water transport and mucin secretion rates. Finally, we hypothesize that we can do little to acutely downregulate mucin secretion during an AE. In Sp. Aim 3, we test whether we can rehydrate the abnormal (dehydrated) mucus that we speculate is characteristic of an AE via inhalation of hypertonic saline (HS), and whether it is safe to deliver HS to a COPD patient with an AE. Thus, we will test this hypothesis by exposing COPD subjects before (to assess safety) and during an AE to inhaled 7% HS and test acute (surrogate) efficacy via measures of mucus clearance and spirometry. This project depends heavily on Project IV (D. Peden) and Project VI (S. Donaldson) for comparative normal and CF data describing mucus composition (hydration, nucleotides+nucleosides, and mucin molecules). The ultimate goals articulated in this Project for the U.S. COPD patient population are to generate a mechanistic understanding of the failure in host defense that produces COPD and test novel approaches to restore host defense aimed at rehydrating the surface of the COPD lung.